1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to an ESD (Electrostatic Discharge) protection element for protecting the inside of an LSI from a surge current and the like.
2. Related Art
When static electricity built up on a machine or human body is applied to an electronic circuit of a semiconductor integrated circuit during manufacturing or in use, there is a possibility that a gate insulating film is damaged by high voltage. Such a phenomenon is known as ESD breakdown. Therefore, most semiconductor devices have a semiconductor element or circuit called an ESD protection element for preventing the inflow of an externally applied surge current, thereby preventing ESD breakdown of a gate insulating film.
Meanwhile, field effect transistors that are basic elements of a semiconductor integrated circuit have been scaled down as performance thereof has been improved. In recent years, it is not uncommon that field effect transistors have thin gate insulating films having an equivalent oxide thickness of about 1 nm. The dielectric breakdown voltage of a gate insulating film is significantly lowered as the thickness of the gate insulating film is decreased, and therefore a surge voltage to be prevented by an ESD protection element (hereinafter, simply referred to as a “protection voltage”) is also lowered as the dielectric breakdown voltage is lowered.
However, it is difficult to arbitrarily control the protection voltage of an ESD protection element. Particularly, in recent years, it has become very difficult to set the protection voltage to a low value appropriate to a very thin gate insulating film.
Against this backdrop, a method which makes it easy to set a protection voltage by using a MOS diode as a protection element has been reported (see, for example, Japanese Patent Laid-open No. 5-67777). However, since this known method is based on a wrong understanding that thin insulating films are not damaged by F-N (Fowler-Nordheim) current, its first concern is to suppress fluctuations in threshold voltage. For this reason, this method uses a normal MOS diode as a protection element. As a result, the resistance of the protection element is high when the protection element is in ON-state so that it is not possible to dissipate voltage quickly. In addition, it is necessary for the protection element to have an insulating film thinner than that of an element to be protected to adjust a protection voltage. However, it is difficult to form such a thin insulating film.
As described above, conventional ESD protection elements have a problem in that it is difficult to set a protection voltage to protect a very thin gate insulating film.